This invention relates to a light amount control of an exposure lamp executed in a copying machine or like image forming apparatus provided with a magnification changing function such as an enlargement mode and a reduction mode.
Conventionally, a copying machine or like image forming apparatus has been designed such that the light from an exposure lamp provided in an optical system is projected onto a document face, and the light reflected by the document face is introduced to a photosensitive member to thereby form an electrostatic latent image on the surface of the photosensitive member.
Regarding an amount of light emitted from the exposure lamp, there has been proposed a system of correcting the light amount at a life size magnification according to a set magnification in an enlargement mode or a reduction mode in order to obtain a desired toner image density (Japanese Unexamined Utility Model Publication No. 63-98539).
In the above prior art, only the magnification is considered as a factor influencing the toner image density, but no consideration is made on other factors including the density of the document and the set exposure amount.
These factors mainly originate in the optical system. For example, the distance between the exposure lamp and the photosensitive member, i.e. the length of an optical path, is fixed independently of the magnification. However, the distance between the exposure lamp and a lens is shortened in the enlargement mode while being lengthened in the reduction mode. Accordingly, the reflectance of the lens is fixed despite the variation in the brightness of the exposure lamp, whereas an amount of light reflected by the lens varies as the brightness of the exposure lamp varies, thereby causing an amount of light transmitted through the lens to vary.
Further, an aperture of the lens differs depending upon the magnification. Moreover, the reflectance, the aperture, and the like differ completely depending upon the type and the construction of a lens unit.
The exposure lamp is required to have the light amount thereof corrected according to the levels of the respective factors influencing the toner image density excluding the magnification.
An optimum light amount of the exposure lamp which will provide a desired toner image density cannot be obtained in the enlargement mode and the reduction mode merely by adjusting the light amount by the same amount corrected at the lift-size magnification. Thus, the conventional image forming apparatus has suffered the incapability of controlling the light amount of the exposure lamp optimally according to the levels of the various factors.